1. Introduction
This invention relates to a method for reducing photoresist reticulation during plasma etching, and more particularly, to plasma etching a reticulation resistant photoresist in a method for the manufacture of a semiconductor device.
2. Description of the Prior Art
Plasma etching is a technique well known in the art and commonly used in the manufacture of semiconductors. An article describing plasma etching is entitled "Plasma Etching In IC Technology" by Kalter and Van der Ven, which appeared at pages 202-210 of Philips Technical Review, Volume 38, 1978/79, No. 7/8.
Radiation sensitive photoresists commonly used in the semiconductor industry are sensitive to different forms of radiation. For example, some photoresists are sensitive to visible light and some to x-rays, while others are sensitive to radiation in the form of beams of charged particles such as electrons. By using a lithographic procedure that involves exposure to the appropriate radiation followed by development in a suitable developer, a photoresist film on a surface can be patterned to form a mask which is resistant to a subsequent plasma etching treatment.
Radiation sensitive photoresists are classified as positive or negative acting. When a positive acting photoresist is exposed to radiation, the exposed parts can be removed selectively because they become soluble in a developing solution while the unexposed parts remain insoluble. With a negative acting photoresist, the exposed parts become insoluble while the unexposed parts remain soluble. In general, positive acting photoresists provide better image resolution than their negative acting counterparts. For this reason, because of a continuing desire for increasing miniaturization, the semiconductor industry has tended to prefer positive acting photoresists for the manufacture of integrated circuits. However, photoresists offer poor resistance to plasma etching as demonstrated by wrinkling or roughening of the photoresist surface--a condition referred to in the art as reticulation. Reticulation is undesirable as it can result in an unacceptable variation in the fabrication of a metal profile.
Various methods have been proposed to counteract the poor resistance of photoresists to plasma etching. One method proposes the use of a layer of photoresist in the range of from about 1.0 to 2.0 micrometers in thickness to compensate for undesirable erosion. However, image resolution is reduced as photoresist thickness increases and high resolution is required for semiconductor manufacture.
Others in the art have devised different plasma etching systems and new photoresist materials in an effort to improve reticulation resistance of a photoresist to plasma etching while retaining the benefit of high resolution. For example, in a paper authored by Harada, "Additives That Improve Positive Resist Durability For Plasma Etching," Journal of the Electrochemical Society; Solid State Science and Technology, Volume 127, No. 2, Feb. 19, 1980, pages 491 to 497, Harada claims that the etch resistance of conventional positive photoresists can be improved by including an additive such as a radical scavenger or a free radical, e.g. 1,1 - diphenyl - 2-picrylhydrazl and galvinoxyl, or a plastics antioxidant such as 2,4,6 -tritert - butyl phenol. Though Harada's technique increases etch resistance, the increase is at the expense of other important photoresist properties such as photoresist sensitivity.
Another method attempted in the prior art to improve the reticulation resistance of an imaged photoresist involving the use of additives is disclosed in U.S. Pat. No. 4,581,321 incorporated herein by reference. In this patent, in addition to the usual photoresist components, the formulation contains an acid catalyzed cross-linking agent and an acid release compound that releases acid such as by photolysis. Upon exposure of the photoresist coating to activating radiation in an image pattern, photolysis results in the release of the acid. The acid catalyzes the cross-linking agent resulting in some cross-linking or hardening of the photoresist. However, the cross-linking that occurs is inadequate to provide sufficient reticulation resistance to the high temperatures generated during plasma etching. Consequently, subsequent to development of the photoresist image and prior to plasma etching, it is necessary to increase the number of cross links such as by a high temperature bake at temperatures of at least 130.degree. C. for, for example, 30 minutes, followed by a deep u.v. (&lt;330 nm) flooding of the resist surface. Though this method does provide a developed photoresist with improved thermal resistance, the additional step of a high temperature bake or u.v. flooding increases processing time and consequently, processing costs. Moreover, during the high temperature bake or the u.v. flood, the photoresist, which is not intended to be a permanent coating over the substrate, becomes firmly adhered to the substrate and insoluble in strippers conventionally used to remove a developed photoresist from a substrate. Consequently, removal of the photoresist from the substrate following plasma etching may be a significant problem when reticulation resistance is increased by a cross-linking mechanism as described in the patent.
Another process for increasing reticulation resistance of a photoresist is disclosed in U.S. Pat. No. 4,600,686, incorporated herein by reference. In accordance with the teaching of this patent, a photoresist coating is formed over a substrate employing materials and processing steps well known in the semiconductor industry. An etch resistant surface layer or skin is formed over the photoresist to reduce plasma attack on the photoresist mask so that, as stated by the patentee, thinner resist films such as less than 0.25 micrometers can be used for increased resolution. The process comprises forming the imaged photoresist layer over the substrate and then sputtering a layer of chromium onto the imaged resist surface. The thickness of the sputtered chromium coating varies typically between 50 and 100 angstroms. Following sputtering of the chromium onto the photoresist coating, the photoresist is chemically reacted with the chromium by heating the photoresist to a temperature of approximately 130.degree. C. for 30 minutes. The skin formed over the photoresist is resistant to subsequent plasma etching. The chromium surface may be etched in a plasma containing one part carbon tetrachloride to one part oxygen in three parts of a carrier gas such as argon or carbon monoxide. Following etching, the patterned photoresist film, together with the skin, are removed using fuming nitric acid. Though the method described in this patent does improve resistance of the resist to plasma etching, the steps of chromium sputtering, baking and nitric acid dissolution of the chromium are required. Such steps increase processing time and expense.